Our recent finding that CD7 was absent from the T cells of a child with Severe Combined Immunodeficiency Disease (SCID) suggests that CD7 plays an important role in the ontogeny of T cells. Furthermore, studies in this patient have suggested that CD7 can influence B cell function. Therefore CD7 plays a critical role in lymphoid development and its absence during early lymphopoiesis leads to combined immunodeficiency. In this unique instance of SCID with CD7 deficiency, we will have a good opportunity to study the molecular and genetic basis of the defect. As little is known about the molecule structure of this molecule, we will seek to generate a cDNA probe for CD7. Three approaches will be used. One method is to isolate the CD7 glycoprotein by affinity chromatography produced with an available monoclonal antibody. The purified protein will be subject to amino sequence analysis and synthetic oligonucleotides deduced from the amino acid sequence will be used to probe a lambda gt10 library, produced from the mRNA of CD7+ Jurkat cell line. Secondly, polyclonal antisera to CD7 will also be used to screen a lambda gt11 expression library, as an alternative to detect for cDNA clone for CD7. Finally, co-transfection of T cell DNA and thymidine kinase positive (tk+) plasmid into tk- L-cells will provide another mean with which to identify and isolate cDNA for CD7. When a cDNA probe becomes available, the genetic defect in this unique case of SCID will then be analyzed. Deletion/insertional mutations will be detected by restriction enzyme fragment polymorphism. Point mutations will be detected by sequence analysis. In order to investigate the functional role of CD7 in T cell ontogeny, a murine model will be sought. This is predicated on the identification of a CD7 homologue in the mouse. The availability of mAb to CD7 and cDNA probes for CD7 will provide means with which a murine homologue for CD7 can be obtained. mAB to CD7 which are made in hamsters will be tested for cross reactivity to a murine homologue. Such an antibody will enable better characterization of CD7 in the mouse. Furthermore, it will be used to immunoselect of CD7+ precursor T cells in normal and nude mice. In vivo functional role of these cells can then be analyzed. Attempts will also be made to induce immunodeficiency with these mAb to murine CD7. The successful outcome of this approach will provide further opportunities to probe the role of CD7 in T & B lymphocyte development during ontogeny.